The present invention relates to methods and apparatuses for measuring the intraocular pressure of an eye, and more particularly to tonometer methods and apparatuses for performing this measurement without touching the eye itself.
Normal pressure within the human eye ranges between about 10 mm Hg and about 20 mm Hg above atmospheric pressure, which ranges between 700 mm Hg and 800 mm Hg at sea level, and which is nominally around 760 mm Hg at zero degrees Celsius. The eye pressure above atmospheric pressure is formally called intraocular pressure, or IOP. The intraocular pressure varies during the time of day by an amount of 3 mm Hg to 4 mm Hg, generally being highest in the morning. It also varies during the course of the year, generally being highest in the winter.
Glaucoma is a disease whereby peripheral vision is lost first, and it is related to elevation of the pressure within the eye to values higher than 21 mm Hg above atmospheric pressure. Such elevated pressure, over long duration, can cause blindness. Glaucoma affects as much as 2% to 3% of the population over the age of 40, and is a leading cause of blindness. The disease can be treated, but not cured, by application of one of a number of drug-therapy regimes. These regimes usually last for the rest of the patient""s life, and require close monitoring and frequent eye-pressure measurements. In cases when the drug treatment is inadequate, laser or incisional surgery may be tried.
Instruments for measuring eye pressure are called tonometers. They are typically not portable, and could be quite expensive. Moreover, they need to be operated by a doctor or trained technician while the patient holds a fixed position with respect to the instrument. To date, there has not been a commercially successful tonometer which can be operated by the patient alone and that is portable and inexpensive, (although it should be mentioned that in April 2001 a new eyelid-contacting tonometer has been introduced, operating on the claimed experimental effect that when an object is pushed against the eye there will be a faint light halo appearing in the eye at the point in time when the external pressure at the area of contact equals the intraocular pressure at that moment.) The consequence of this is that frequent measurements of the patient""s eyes are typically not made in order to determine the full range over which the patient""s IOP varies. Because of this, doctors end up using a less than optimal application of the drug-therapy regimes since they do not have enough measurement data to fine-tune the regimes. In addition, a doctor may fail to suspect and diagnose a patient""s glaucoma because the tonometer measurement may have been taken at a time when the patient""s IOP was at its lowest point in the range during the measurement.
Most of the tonometers described above operate by pressing an area of the eye by a known force and then measuring the resulting displacement, or by pressing the area of the eye by a known displacement and measuring the force required to do so. The former approach may be conducted by an xe2x80x9cair-puffxe2x80x9d tonometer, which blows a puff of air toward the eye at a known force. Either of the above approaches may be conducted by a contact tonometer, which has a plunger that physically contacts the eye. Air-puff tonometers are uncomfortable, and contact tonometers require that the patient""s eyes be anesthetized.
To address these problems, much research work has been done in the area of vibration tonometers. These tonometers apply vibrations to the eye, such as by a loud speaker or by a vibrating element contacted to the eyelid, vary the frequency of vibrations to find the maximum amplitude vibration of the eye (called the xe2x80x9cresonance pointxe2x80x9d), and compute the IOP based on the frequency of maximum amplitude vibration. These tonometers are based on the assumption that the human eye can be modeled as a spherical body of water held together by the surface tension of the water (the so-called xe2x80x9cwater dropxe2x80x9d model). Such a body of water has a plurality of vibratory modes n=1, 2, 3, . . . , each of which has a corresponding natural frequency, or resonant frequency fn,res, at which the surface vibrations of the water drop are at maximums. The value of each resonant frequency depends upon the difference in pressure, xcex94p, between the interior of the water body and the external atmosphere, as provided by the following equation:       f          n      ,      res        =            λ      n        ·                            Δ          ⁢                      xe2x80x83                    ⁢                      p            /            ρ                                      π        ·        a            
where:
xcexn is the eigenvalue of the n-th mode, having approximate values of 1.0, 1.94, 3.0, and 4.18 for value of n=2 through n=5;
xcfx81 is the fluid density;
xcex1 is the radius of the sphere, and
xcfx80 is a constant equal to the ratio of the circumference of a circle to its diameter (3.14159. . . ).
As applied to the eye, the pressure difference xcex94p has been equated to the eye""s intraocular pressure, as the IOP is defined as the pressure in the eye that is above atmospheric pressure. Many prior art approaches have used the above to model the eye.
However, it is important to note that the water-drop model predicts a zero value for each resonant frequency at 0 mm Hg of intraocular pressure. That is to say that at 0 mm Hg, f1,res=0, f2,res=0, f3,res=0, f4,res=0, etc.
As indicated by U.S. Pat. No. 5,865,742 to Massie (Non-Contact Tonometer), the use of this model for the measurement of intraocular pressure (IOP) has not met with success. The following quote from U.S. Pat. No. 5,865,742 points to some reasons for lack of success:
xe2x80x9cOne additional type is the vibration tonometer, first patented in the 1960""s (U.S. Pat. Nos. 3,192,765 and 3,882,718). In this device, it is proposed that the response of the eye to a vibrational excitation will be a measure of the IOP. The proposed exciters include very low-frequency sound and mechanical plungers. However, it is likely that the vibrational frequencies of the eye are affected by many factors not related to the IOP. It is, in fact, expected that the actual resonance spectrum of the eye would be dictated more by the connective tissue than by the IOP. All of these factors may be the reason why no commercial use of the vibration tonometer has been disclosed even though its development has been attemptedxe2x80x9d (Massey patent, column2, lines 50 to 62).
A thorough theoretical background going beyond the simple water balloon model is provided by xe2x80x9cA Nonlinear Modal Frequency Response Analysis of the Pre-stressed Human Eye by the Finite Element Methodxe2x80x9d by K. C. Henderson (submitted in partial fulfillment of the requirements for the degree Master of Science, University of Rochester, 1995) with experimental results described in a concurrent associated thesis for the same degree at the same university: xe2x80x9cIntraocular Pressure Measurement Using Resonance Detectionxe2x80x9d by K. S. Bhella.
While vibration tonomoters offer the possibility of inexpensive and convenient measurement tools, they have not met with successful implementation, and consequently have not met with commercial success. The present invention is directed to providing a vibration tonometer that does not touch the surface of the eye and that provides accurate and reliable results, and which is affordable by home users.
In making their invention, the inventors have recognized that the xe2x80x9cresonant frequenciesxe2x80x9d computed by the water-drop model do not account for the damping by the surrounding tissue and connective muscles, and that the frequencies computed by the model are, in reality, undamped natural frequencies that do not take into account the damping. The inventors have further determined that nearly all of the prior art vibration tonometers have measured each water-drop xe2x80x9cresonant frequencyxe2x80x9d of the eye by finding a frequency at which an area of the eye""s sclera undergoes maximum vibratory displacement when excited by an excitation source, and that this resonant frequency is below the natural frequency predicted by the water-drop model. The inventors have further found that the detection of the water-drop xe2x80x9cresonant frequenciesxe2x80x9d is obscured due to the damping of the surrounding tissue and connective muscles.
In making their invention, the inventors have discovered that the sclera of the eye, which is the outer shell of the eye, has classes of undamped natural frequencies that are not predicted by the water-drop model, with each undamped natural frequency being associated with a corresponding vibratory mode of the shell formed by the eye""s sclera and cornea. The value of each natural undamped frequency depends upon the intraocular pressure, increasing in value as this pressure increases. At each level of intraocular pressure, the natural frequencies in these classes are different in value from the damped and undamped natural frequencies of the water-drop model, and can be measured with less interference from the surrounding tissue and connective muscle. One characteristic of one of these classes of natural frequencies of the sclera is that their values approach a non-zero value when the intraocular pressure goes to zero mmHg. As another characteristic, when the curves of these frequencies versus intraocular pressure are extrapolated toward a value of zero frequency, the curves tend to converge to a common negative intraocular pressure value. These characteristics are different from the undamped natural frequencies of the water-drop model, all of which converge to a value of zero when the intraocular pressure reaches zero.
The inventors have further found that there is a band of vibratory frequencies around each undamped natural frequency which have similar properties as the corresponding undamped natural frequency, and which may be similarly utilized in the present invention. Each such band of vibratory frequencies includes the undamped natural frequency and the corresponding damped natural frequency, and is associated with the same vibratory modes associated with the undamped natural frequency. When a sinusoidal vibratory force with a frequency within such a frequency band is applied to a spot on the eye, such as by sonic pressure waves or ultra-sonic pressure waves, or is otherwise coupled to the eye, such as by a mechanical transducer contacted to tissue or bone near the eye, portions of the sclera and cornea surrounding or near the excitation spot vibrate in response with the same frequency. These portions are called xe2x80x9canti-nodes.xe2x80x9d Other portions of the sclera and cornea, called xe2x80x9cnodes,xe2x80x9d remain relatively stationary while the anti-nodes vibrate. Each vibratory mode has a corresponding set of nodes and anti-nodes arranged in a corresponding pattern when the mode is excited. In general, an anti-node comprises a polygonal area covering a portion of the sclera and/or the cornea, and a node may comprise a great circle of a sphere, a small circle of a sphere, a line, or a point, all of which are located on the pseudo-spherical surface formed by the sclera and the eye. In general, the number of nodes and anti-nodes increases as the order of the mode increases. The band of vibratory frequencies associated with a corresponding vibratory mode is defined as the contiguous set of frequencies, which cause at least one node, and two anti-nodes of the mode""s pattern to be present. For excitation frequencies that are between two separate but adjacent vibratory frequency bands, no regular pattern of nodes and anti-nodes is present.
Accordingly, the present invention encompasses methods and apparatuses for estimating the intraocular pressure of an eye, which is the difference between the pressure within the eye and the atmospheric pressure. Broadly stated, methods according to the present invention comprise measuring a first vibratory frequency of an associated vibratory mode of the sclera of the eye at an unknown intraocular pressure value, the first vibratory frequency having a value that varies as a first function of the eye""s intraocular pressure. The first function has a form which extends or extrapolates to a non-zero frequency value for a zero value of intraocular pressure and to a zero frequency value for a negative value of intraocular pressure. Methods according to the present invention further comprise comparing the measured frequency value to one or more known values of the first vibratory frequency measured at corresponding known intraocular pressures to estimate value of the unknown intraocular pressure. Preferred embodiments of the present invention include measuring one or more additional vibratory frequencies of the cornea or the sclera of the eye at the unknown intraocular pressure value, and comparing their measured values to known values of the additional measured vibratory frequencies to estimate value of the unknown intraocular pressure. In preferred embodiments of the present invention, each of the vibratory frequencies comprises a corresponding undamped natural resonant frequency of the sclera or the cornea.
The present invention is in contrast to all prior art methods known to the inventors for estimating the intraocular pressure of the eye by vibratory excitation in that the present invention detects and measures vibratory frequencies that have non-zero values for a zero value of intraocular pressure.
The present invention is able to measure the vibratory frequencies of the eye""s cornea or sclera more reliably and accurately than the prior art methods of measuring the xe2x80x9cresonant frequenciesxe2x80x9d of the water drop model. The present invention is also able to do so without directly contacting the surface of the eye or shooting annoying puffs of air at the eye. Furthermore, the present invention is expected to be able to perform the measurements with relatively inexpensive components.
Accordingly, it is an object of the present invention to enable the measurement of the IOP of a patient""s eye with an inexpensive and relatively portable tonometer.
It is another object of the present invention to provide a tonometer which can be operated by the patient at home or other places that are not at the doctor""s clinic.
It is yet another object of the present invention to enable the patient""s eye to be measured more frequently, and to thereby enable better health care.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention, the accompanying drawings, and the appended claims.